The present invention relates to velocity well logging of the acoustic type and particularly to such well logging by a process or system which involves lowering the acoustic well logging probe into a borehole in an earth formation to be investigated and measuring of the transit time of acoustic waves between points in the formation.
It is common practice to measure the difference in travel time of the compressional portion of an acoustic wave between a transmitting transducer or transmitter of acoustic waves and, for example, two receiving transducers or receivers positioned at predetermined different spacings from the transmitter in the well logging tool or probe. Successive acoustic signal pulses or waveforms are generated by the transmitter and travel through the borehole fluid to the formation, through the formation, and then back through the fluid to the receivers spaced along the probe to generate electrical signals in the receivers. Each of these electric signals consist of undesired baseline noise in addition to an electrical analog of the acoustic wave received from the transmitter. The latter is made up of a succession of wave arrivals, signal parts or half-cycles. The desired information is in the time of arrival of the parts or half-cycles. The earlier the arrival or half-cycle, the less is the possibility of error due to distortion or arrivals from multiple paths through the formation.
A common arrangement for determining the time of arrival of the acoustic waves at the receivers involves use of detection circuits for the receiver signal waves, which circuits discriminate in accordance with signal amplitude, in an effort to identify the same signal arrival or part in the signal generated in each receiver from each succeeding acoustic signal. This type of arrangement is usually provided with adjustable bias control so that adjustment may be made to trigger the signal arrival detection circuitry above the noise level as operating conditions very. However, raising the trigger amplitude for such circuitry above the baseline or zero value can introduce errors into the measurement if the amplitude of the detected arrivals that are employed in the measurements are not the same. Thus, if one arrival or half-cycle portion has a greater amplitude than a corresponding arrival or half-cycle signal portion crossing the zero potential base line at the same time, the former arrival or half-cycle signal portion will attain the trigger amplitude at an earlier corresponding time in the half-cycle than the latter. This condition is known as "waveform stretch" and can cause a discrepancy of up to about four microseconds in the measured arrival time in a system employing commonly used frequencies and acoustic pulse rates. This type of system also has the disadvantage in that dropping of the amplitude of a particular waveform portion or half-cycle in the signal from one receiver below the trigger threshold results in missing entirely that half-cycle, while the corresponding amplitude in the corresponding half-cycle in the other receiver does not drop, and thus is detected, introducing a measuring error known as "cycle skip". Such "cycle skip" can cause an error either reducing or increasing the measured time depending upon which receiver half-cycle is misused. Such error may amount to up to about 30-35 microseconds in a conventional system. If the trigger amplitude of the detecting circuitry is lowered too close to the baseline to avoid the other measuring errors, the circuitry could trigger on "noise" in the received signals to introduce another type of serious error in the measurements.
In one type of an acoustic well logging system, such as disclosed in U.S. Pat. No. 2,963,646 to Hicks et al, transition points in the received signal waves, such as the zero crossing transition point, are used as indications of the time of arrival of acoustic waves at the receiver. The system involves arranging for a gating circuit to respond to the second arrival or half-cycle of the acoustic-responsive signal in a receiver to provide a gate pulse for a period of time including the time of zero crossing between the second half-cycle and the third half-cycle of the received signal wave. Additionally, a short pulse is produced in response to the occurrence of the aforementioned zero crossing. The two pulses are applied to a coincidence circuit to provide an operating signal which starts operation of a monotonically varying function generator at the time of zero crossing. Similarly, a second operating signal indicative of the time of the corresponding zero crossing of the signal in the second receiver is operative through a blocking oscillator and additional circuitry to obtain the amplitude of the function generator output for recording. This system follows what the patent states is the usual practice of using the second half-cycle of the receiver signal wave to condition the measuring circuitry because this half-cycle is usually of larger magnitude than the first pulse and hence easier to track.
Another prior art acoustic well logging system uses the previously discussed procedure of detecting points where signal wave amplitude reaches a threshold value as an indication of time of arrival, but compares the difference in measured arrival times at two receivers in two succeeding measurement sequences. This system, disclosed for example, in U.S. Pat. No. 3,900,824 to Trouiller et al applies a substitute valid time value obtained in a previous measuring sequence to the recorder in place of the difference in arrival times measured in the current sequence if the latter difference varies from the difference determined in the preceding sequence by more than a predetermined amount.
The present invention takes into consideration that the earlier arrivals or half-cycles of an acoustic signal at a receiver furnish a more accurate indication of travel time of the acoustic wave from the transmitter than later arrivals or half-cycles. It also recognizes that apparatus for measuring the earlier arrivals is more likely to provide invalid time measurements than apparatus for measuring later arrivals or half-cycles. A novel and unobvious method and apparatus are disclosed for using time measurements involving earlier arrivals or half-cycle signals at acoustic well logging receivers for measuring sequences where such measurements are valid; substituting other measurements, such as those obtained from generally less accurate but more reliable (more likely to be valid) later arrivals or half-cycles; for measuring sequences where invalid results are obtained in the effort to use the earlier arrivals. Moreover, in embodiments of the present invention the sequence of arrival of the first detected positive and negative half-cycles at each receiver are compared to reduce the chance of error due to cycle skip.
It is thus an object of the present invention to provide a novel and improved method and apparatus for acoustic well logging wherein transition points occuring in corresponding early arrivals or parts of signal waves in two receivers are utilized only if the difference in time therebetween meets a standard indicative of a possible valid measurement.
It is another object of the invention to provide a novel method and system of acoustic well logging wherein determinations of arrivals of signal waves of two receivers are made at what are deemed to be two corresponding sequential sets of points and the determination made from the set earlier in sequence is utilized only if found to be valid.
It is yet another object of the invention to provide a novel method and system of acoustic well logging wherein determinations of zero crossing points of signal waves of two receivers are made at two sets of sequential corresponding points and the set which appears in the earliest parts of the waves that is truly representative of the corresponding zero crossings is recorded.
These and other objects and features of the present invention will become apparent from the following drawings and detailed description of the preferred embodiments of the present invention.